Devices of this type can be used for example in wavelength-division multiplexing communications networks, for extraction from the line of information for a certain user and re-insertion in the line of information generated by the same user. Indeed, in such networks information is usually broadcast and selected, i.e., information from each user is broadcast to all others, and each user, in order to extract information of concern to him, must select a certain wavelength from the set of wavelengths present on the line. Another application is found for example in the routing nodes of reconfigurable optical networks, to re-route certain information streams due to changed conditions in the traffic or to by-pass a fault downstream from a node.
For the implementation of devices of this type, the use of tunable acousto-optical passband fitters based on TE/TM polarization converters has been proposed. Examples of these are found in the papers "A fully transparent fiber-optic ring architecture for WDM networks", by M. I. Irshid and M. Kavehrad (Journal of Lightwave Technology, Vol. 10, No. 1, January 1992, pages 101-108) and "Wavelength-division-multiplexing add/drop multiplexer employing a novel polarisation independent acousto-optical tunable filter" (Electronics Letters, Vol. 29, No. 10, May 13, 1993, pages 905-907).
In particular, the device described in the latter paper is a two-input, two-output device including a pair of optical circulators, one of them being connected to the line and the other to a local processing device, and a polarizing beam splitter-recombiner and a TE/TM acousto-optical converter connected between the two circulators. The two ports of the converter are connected to the circulators through respective sections of polarization maintaining optical fibers, one of which rotates by 90.degree. the polarization of the beam traversing it. The radiation in the line, usually elliptically polarized, is transferred from the first circulator to the splitter-recombiner, that splits it into two beams with orthogonal polarization. The beam with TE polarization is reflected and sent to one of the ports of the converter; the beam with TM polarization, which is transmitted, goes into the fiber section which rotates by 90.degree. its polarization, thus likewise arriving with TE polarization at the other port of the converter. The two beams traverse the converter in opposite directions. If the wavelength of the two beams is one of those the filter is tuned to, the beams leave the filter with TM polarization and are again sent to the splitter-recombiner. The beam traversing the fiber section which rotates the polarization reaches the splitter-recombiner with TE polarization and is reflected back towards the second circulator; the other beam maintains its TM polarisation and is transmitted towards the second circulator. The recombined beam is then sent to the local processing device. For any other wavelength, the polarization state of the two beams is left unchanged by the filter and the two beams are again sent from the splitter to the first circulator, so that the recombined beam is re-inserted into the line. Similarly, a radiation introduced through the second circulator is transferred to the line through the first circulator if its wavelength is one of those the filter is tuned to.
The known device allows extraction and re-insertion of several carriers at the same time; however, it has a few limitations. Its main shortcoming is the low selectivity of tunable acousto-optical converters, which can only separate wavelengths if these are at least a few nanometers apart, thus limiting the system capacity relative to the available bandwidth. Another shortcoming is that the device can introduce selective attenuation of the two field polarization components, so that the polarization of the output signal may differ from that of the input signal. Moreover, acousto-optical polarisation converters are not easily found on the market.